Coal-bed gas reservoir is quite different from ordinary sandstone or carbonate reservoir, and has characteristics such as high adsorbability, low permeability, and susceptibility to compression and fracture, etc. Owing to those characteristics, the coal bed is subjected to much more severe injuries than an ordinary reservoir in the coal-bed gas well drilling process, and the injuries of the coal-bed gas reservoir have direct influence on the desorption, diffusion, migration, and subsequent drainage and mining. Therefore, close attention should be paid to the coal-bed gas reservoir injury problem. A coal bed is mainly subject to the following injury factors owing to its characteristics:    (1) The development of coal bed pores and fissures provides an objective condition for the solid phase and liquid phase in external fluids to invade into the coal bed;    (2) The pore-fissures system and the nature of macromolecular organic substance of coal create a condition for adsorption of water and high molecular polymers in external fluids, which has influence on the permeability of the coal bed;    (3) The water in the coal bed may have chemical reactions with external fluids easily, and the resultant compound precipitates may block the fissures in the coal bed;    (4) Since most coal reservoirs are in an under-compacted state, leakage may occur easily during drilling and cause serious injuries to the coal-bed gas reservoir;    (5) The coal bed is highly sensitive to stress, and the stress damage resulted from many factors in the drilling process causes decreased reservoir permeability;    (6) The coal rocks have low mechanical strength, and the coal powder generated in the well drilling process may block the pores and fractures in the coal bed and thereby result in damages to the solid phase in the coal bed.
The Patent Document No. CN104910877A has disclosed a film former composition for drilling fluids, which contains at least one modified phenol-formaldehyde resin, at least one hydrophobic monomer, at least one acrylate, and at least one epoxy resin. The film former obtained in the patent application can achieve relatively low coal bed leakage at higher temperatures; however, the patent application hasn't mentioned any coal-bed gas reservoir protection feature of the film former.
Drilling fluids are reputed as the blood for well drilling. Good drilling fluid techniques are one of the important guarantees for safe, high-quality, efficient, and quick well drilling production. Drilling/completion fluids are required in the exploitation of coal-bed gasses. Effective reservoir protection is an important guarantee for a high rate of oil and gas recovery, so as to attain an optimal negative skin coefficient. If the drilling/completion fluid is designed or used inappropriately in the well drilling/completion process, the liquid and solid in the drilling fluid may intrude into the oil reservoir and have physical and chemical reactions with clay and other minerals in the oil and gas layer. Consequently, the permeability in the oil layer in the immediate vicinity of the well may be decreased severely, and the resistance against oil and gas flow towards the bottom of the well may be increased, resulting in reduced oil yield. The injuries to the reservoir have critical impacts on effective exploitation of the reservoir. The major forms of reservoir damages happened in the coal-bed gas well drilling process include:    (1) The drilling fluid is absorbed by the coal rocks, resulting in decreased permeability of the coal rocks;    (2) The solid particles in the drilling fluid fill and plug the channels formed by fissures;    (3) A polymer drilling fluid invades into the coal bed and causes clay flocculation and blockage under the adsorptive action of high molecular polymers, as well as clay swelling and blockage under the action of carboxyl hydration, and thereby results in decreased permeability of the coal bed;    (4) The drilling fluid interacts with the formation water and generate solid precipitates, which result in blockage of channels formed by pores.
Production factors in the well drilling process, such as excessively high pressure of the drilling fluid column, reservoir bed soaked in the drilling fluid for an excessively long time, and excessively high pressure, etc., may also cause injuries to the coal bed. Furthermore, well cementing operation and reservoir reformation measures taken to improve the yield (e.g., hydraulic fracturing) may also cause injuries to the coal-bed gas reservoir in different degree.